An RF switch is an electrical on-off switch for an RF signal. When the RF switch is turned “on”, it functions to have an RF signal, applied to its input terminal, normally transmitted to its output terminal, and when the RF switch is turned “off”, it functions to hinder the RF signal from being transmitted to the output terminal. This “on” and “off” operation of the RF switch is changed depending on the polarity of a DC-controlled voltage that controls the RF switch.
This RF switch has a variety of types. The most basic types can include a Single-Pole/Single-Throw (SPST) switch having one RF signal input and one RF signal output and a Single-Pole/Multiple-Throw (SPMT) switch having one RF signal input and several RF signal outputs.
This electrical switching of the RF switch is performed by a diode, preferably, an RF switching diode known as a PIN diode. The PIN diode is a constitutional element that plays a pivotal role in an electrical circuit of the RF switch. As well known to those having ordinary skill in the art, the PIN diode is a semiconductor element having two terminals. In the PIN diode, current flows only in one direction from the anode terminal (anode side) to the anode terminal (cathode side) like other diodes, and when a positive voltage is applied to the anode, the diode is forward biased, so the current flows.
When the diode is biased so that the current can flow therethrough, that is, when the diode is forward biased, the diode provides resistance that is very low or almost zero so that the current can flow therethrough. This state is called an “on” state. When the diode is biased in an opposite direction, that is, when the diode is reverse biased, the diode provides infinitely high resistance to thereby form an open circuit, so the current cannot pass through the diode normally. This state is called an “off” state.
The diode requires a predetermined time when one state shifts to the other state according to a change in the voltage. This characteristic pertaining to the diode is called a transition time. To change the state of the diode, a new voltage for biasing the diode to another state must be applied to the diode during a minimum transition time of the diode.
An AC signal, such as an RF voltage added to a reverse-biased DC-controlled voltage, does not change the state of the PIN diode. This AC signal has a sufficiently high frequency. Thus, when the duration of the voltage swings or peaks in the signal does not satisfy a minimum time required for the transition from the “on” state to the “off” state of the diode, the state of the PIN diode is not changed. However, the state of the diode can be changed by changing the polarity of the DC-controlled voltage in order to forward bias the diode, so that the current, including AC, can flow through the diode.
Further, when the diode is forward biased, an AC signal added to a forward-biased DC-controlled voltage does not change the state of the diode as long as it has a sufficiently high frequency, in the same manner that the diode is reverse biased. Meanwhile, if an AC voltage is too high, the added signal can exceed the breakdown voltage of the diode and break the diode. Thus, in the PIN diode, the breakdown voltage of the diode must be selected not to exceed an added AC signal.
Further, in constructing the RF switch, a shunt RF switch is advantageous in employing an electrical characteristic of the PIN diode. A PIN diode is branched and placed on an RF transmission line and then reverse biased by a control voltage. Thus, the diode serves as an open circuit, so an RF signal is propagated to an output terminal of the diode along the transmission line.
However, if the diode is forward biased so that the current flows therethrough, it provides a path having very small impedance with respect to the RF signal. The RF signal forms a shunt path from the transmission line to the ground via the diode and, therefore bypasses the transmission line to the output terminal. Hence, the RF signal does not pass through the transmission line. FIGS. 1 to 3 are examples showing a conventional RF switch. There are illustrated RF switches including PIN diodes 103, 104, 105 and 106 between two terminals 101,102 in various ways.
Meanwhile, in the case of a TDM (Time Division Multiplexing) transmission/reception system in which an RF signal is transmitted and received through one antenna, a SPDT (Single-Pole/Double-Throw) type RF switch, which can switch a transmission stage and a reception stage, is required. In this system, RF switches placed at the end and first stages require such characteristics as 1) high linearity with respect to a high power, 2) low insertion loss, 3) high isolation, 4) short switching time, and so on.
However, the conventional RF switch can be miniaturized since it is fabricated in a PIN diode switch form using a H-MIC (Hybrid-Microwave Integrated Circuit) technology, but has problems in that the fabrication process is complicated, there are limitations in the use of a high power, such as a relay, and the design of a specific dual band.
FIG. 4 is an example showing another conventional RF switch of a SPDT structure employing a PIN diode. An RF switch 200 is a PIN diode switch of a surface mounting type STDT structure for solving the above problems, and includes a transmission line 205 having an electrical length of −90 degrees with respect to PIN diodes 203, 204 between two terminals 201, 202. Here, as described above, if a forward bias is applied to the RF switch, the PIN diode 203 has low impedance close to short. As the electrical length of the transmission line 205 is set to −90 degrees, the impedance of the terminal 202 with respect to the terminal 201 becomes infinite. Consequently, a signal input through the terminal 201 is introduced to a ground 206 through the PIN diode 203, which is connection in parallel to the ground, but is rarely introduced to the transmission line 205. In other words, the PIN diode 203 becomes an “off” state. As described above, power loss can be minimized by employing the PIN diode and the RH transmission line 205 (that is, a ¼ line of a guided wavelength).
This RF switch has a high linearity and a high degree of isolation even at a high power, and a short switching time. However, the RF switch is problematic in that it becomes bulky when designing a low frequency band since the transmission line having the electrical length of −90 degrees is employed and it has a limitation in the use of a specific dual band.